1. Field of the Invention
The present invention relates to a surface treatment for a paved surface and the equipment for applying the asphalt binder and aggregate to accomplish this treatment. More specifically, the present invention is a seal coat process that uses substantially synchronous multiple applications of asphalt binder and aggregate in a single pass of the equipment over a road surface.
2. Description of the Related Art
Asphalt concrete deteriorates over time through the effects of air and water damaging the flexibility of asphalt cement and the bond of asphalt cement to the aggregate in the asphalt concrete mix. To delay the damaging effects of air and water, surface treatments are placed on top of asphalt concrete to seal the voids. This decreases the exposure of the asphalt binder in the asphalt concrete to air and water.
One of the most common and cost effective surface treatments is known as a chipseal. A chipseal involves spraying an asphalt binder (usually asphalt emulsion but can be asphalt cutbacks or hot asphalt cement) from a distributor truck onto the surface of the pavement. Soon after application of the asphalt binder to the road surface, aggregate is applied by a chip spreader vehicle.
While the most cost effective seal coat available, chipseals can have problems keeping the aggregate adhered to the road. Loose aggregate causes significant windshield damage, and many agencies have discontinued the use or restricted the use of chipseals to low traffic count roads. The main cause of premature aggregate loss is due to low embedment of the aggregate into the asphalt binder. There are multiple variables which contribute to low aggregate embedment including, but not limited to, irregular existing asphalt pavement texture, rock dimension changes, weather, delay in getting the rock applied to the asphalt binder, and traffic load. Another cause of low aggregate embedment is excess dirty fines coating the aggregate and not allowing it to adequately bond to the asphalt binder.
The obvious solution to aggregate loss in chipseals would be to increase the amount of asphalt binder applied to the existing pavement surface to increase embedment. However this approach creates a “bleeding” problem where the asphalt binder is pushed to the surface of the chipseal causing a rich asphalt surface. A “bleeding” or “flushing” chipseal deteriorates the aesthetics of the chipseal as well as potentially lowering the skid resistance of the finished chipseal.
Other attempts have been made to improve aggregate retention. The methods tried to date include precoating the aggregate with asphalt binder before shipping it to the construction site, applying choke stone after the chipseal is constructed to attempt to interlock the chipseal aggregate, and the use of fog seal applications several days after the chipseal is constructed to add extra asphalt to the surface. These approaches have shown improvement in aggregate retention. However, better methods of providing a surface treatment to a surface are still needed.
Also, paving vehicles for applying the asphalt binder and aggregate in a single pass of the vehicle have been employed, such as the paving vehicle which is the subject of U.S. Pat. No. 6,805,516 by James J. Barnat et al. and which is incorporated herein by reference. This patent teaches a roadway paving system and paving vehicle for applying a single layer of asphalt binder and thereafter applying a single layer of aggregate in a single pass with the paving vehicle in continuous fashion without driving on the freshly paved surface. By applying the aggregate to the layer of asphalt binder shortly after the binder is applied to the roadway, aggregate retention is improved. Also because both the binder layer and aggregate layer are applied in a single pass of the paving vehicle behind the rear wheels of the vehicle, the layers of binder and aggregate are not disturbed by the tires of the paving vehicle.
This type of paving vehicle needs to be continually supplied with binder and aggregate as it operates. Thus, specialized supply trucks, such as the one taught in U.S. Pat. No. 6,776,557 by James J. Barnat et al., are required to supply binder and aggregate to this type of paving vehicle as the paving vehicle is operating. The teachings of both U.S. Pat. No. 6,805,516 and U.S. Pat. No. 6,776,557 are incorporated herein by reference.
However, it would be desirable to be able to apply multiple layers of binder and aggregate in a single pass of a paving vehicle. Also, it would be desirable if the layers of binder thus applied could be of two different types of binder material. Further, it would be desirable if the layers of aggregate thus applied could be of two different types of aggregate material. The present invention addresses this need by providing a paving vehicle and supply truck and method for applying multiple layers of asphalt binder and aggregate in a single pass of the paving equipment over a roadway.